For their lower power consumption and less view-angle dependence, organic EL (electroluminescent) displays have been expected to become next-generation displays or luminescent devices, but have the problem of being susceptible to degradation by moisture or oxygen in the atmosphere. To overcome this drawback, organic EL devices in the display panel are sealed with a sealing member.
Currently, there are two different sealing methods of organic EL devices: a method called “frame sealing,” and a method called “cover sealing.” Frame sealing refers to a method in which a sealing cap is placed above organic EL devices formed on the substrate and a sealing material is applied around the periphery of the sealing cap in such a way as to seal in the organic EL devices (see, e.g., Patent Document 1). The sealing cap is a plate-like member of certain shape, such as a glass plate or stainless steel plate. This method is less productive because processing of a plate-like member is highly laborious. Moreover, the sealing cap is susceptible to curling due to the presence of space between the sealing cap and the organic EL devices. Thus, unfortunately, the frame sealing method has been less applicable to the manufacture of larger organic EL panels.
Cover sealing, a method which may overcome the above-described problem associated with frame sealing, refers to a method in which a sealant composition is applied in the space between the sealing plate and substrate and between the sealing plate and organic EL devices, thereby encapsulating the organic EL devices with the sealant composition (see, e.g., Patent Document 2). This method has the advantages of being highly productive as it can dispense with providing a sealing cap, as well as avoiding curling of the sealing plate as no space is left under the sealing plate.
Sealants used for cover sealing need to have high refraction indices (the difference in refraction index between the sealant and transparent cathode electrode should be small) because they are applied in a space formed between organic EL devices and sealing plate. This particularly applies to top emission devices. The reason for this is that when the sealant used has a low refraction index, total light reflection occurs between the cathode and the sealant thus resulting in low light extraction efficiency in the organic EL devices.
It is also required that sealant compositions from which sealants for cover sealing are made exhibit small curing shrinkage. This is because high curing shrinkage leads to generation of small gaps between the cured sealant and substrate due to internal stress, which in turn reduces adhesion and moisture impermeability.
Moreover, sealants directed to cover sealing are required to have a certain level of heat resistance because low heat resistance results in less reliable organic EL panels.
It is also required that sealant compositions for the sealants for cover sealing be liquid around room temperature. If the sealant composition is not liquid around room temperature, it results in the inconvenience of having to melt the sealant composition by heating upon sealing of organic EL devices. Heat treatment causes strains in the display parts and thus sufficient sealing may fail. Moreover, heat treatment promotes curing reactions in the sealant composition thus facilitating generation of viscosity variations.
As resin compositions suitable for optical applications, epoxy resin compositions are proposed which contain an epoxy resin having a fluorene skeleton, and an acid anhydride (see, e.g., Patent Document 3). These epoxy resin compositions are considered to have high heat resistance as well as high transparency for the presence of the fluorene skeleton in the molecular structure of the epoxy resin. From their compositions, they are considered to give a cured article having a refraction index as high as about 1.63. These epoxy resin compositions, however, have high softening points and thus are solid at room temperature; therefore, the use of the epoxy resin compositions as sealant compositions has met with the problem of poor workability.
As adhesive compositions suitable for bonding of optical parts, photocurable adhesive compositions have been proposed which contain a thiol compound and an epoxy compound (see, e.g., Patent Document 4). These photocurable adhesive compositions are considered to have high refraction indices for their high sulfur content. Further, the photocurable adhesive compositions have high softening points because they are free from any rigid molecular structure like fluorene skeleton, thus exhibiting excellent workability at room temperature. Unfortunately, however, they have met with the problem of low heat resistance.
As resins suitable for lens applications, sulfur-containing urethane resins are proposed which contain a polyisocyanate compound and a thiol compound (see, e.g., Patent Document 5). After cured, these sulfur-containing urethane resins are considered to have high refraction indices for their high sulfur content, as well as a certain level of heat resistance for the presence of the polyisocyanate compound. Moreover, for their low softening point, the sulfur-containing urethane resins are considered to exhibit excellent workability at room temperature. However, unlike epoxy resins, the sulfur-containing urethane resins show large curing shrinkage when cured and thus are not suitable for applications including sealants.
Optical instruments and precision instruments, including liquid crystal displays, suffer a breakdown or device degradation due to moisture from the atmosphere. Thus, less moisture permeable sealants are proposed which protect these optical devices and precision devices against moisture (see, e.g., Patent Document 6).    [Patent Document 1] Japanese Patent Application Laid-Open No. 11-45778    [Patent Document 2] Japanese Patent Application Laid-Open No. 2001-357973    [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-41925    [Patent Document 4] Japanese Patent Application Laid-Open No. 2004-35857    [Patent Document 5] Japanese Patent Application Laid-Open No. 02-270869    [Patent Document 6] Japanese Patent Application Laid-Open No. 10-60397